In the building construction industry there exist many standards that are adopted into codes and by extension become local law. These codes govern mechanical and electrical systems, fire protection and life safety, structural, space, and envelop requirements, and energy conservation (Energy Code).
The Energy Code dictates requirements of a building envelop, Heating Ventilation and Air Conditioning (HVAC) systems, water heating, electrical power, and electrical lighting. Within this code are several references to the climate zone the building is being built in. Typically, the climate zones have been defined based on historical meteorological data collection of temperatures, humidity, snow fall, rain fall and other such weather occurrences. The Energy Code uses the climate zone to determine the materials and systems that are required to result in the best investment versus operating cost. Some building systems that are static such as the insulation and vapor barriers can be chosen and installed in the building to protect against extreme cold, heat, and humidity. Other systems such as air conditioning economizer systems are dynamic and have to be adjusted in the field rather than set in the factory in order to achieve the requirements of the Energy Code. The controls of these dynamic systems have to be parameterized such that they function to achieve highest operating energy efficiencies. If the controls are improperly set, they can reduce energy savings or in many cases increase energy use.
Within the Energy Code there are detailed requirements of the set points of operation of specific systems. In the case of air conditioning economizer systems, systems that use outdoor (outside) air to cool the building when conditions are suitable, the parameters of the control determine when air should be brought into a building and when it should not. For example, the specific climate zone setting as defined by the ASHRAE 90.1-2007 for the economizer system is shown in Table 1 below.
TABLE 1Required High Limit (Economizer Off When):Sensor TypeClimate ZonesEquationDescriptionFixed Dry1b, 2b, 3b, 3c, 4b, 4c, 5b, 5c, 6b, 7, 8TOA > 75° F.Outdoor air temperature exceeds 75° F.Bulb5a, 6a, 7aTOA > 70° F.Outdoor air temperature exceeds 70° F.All other zonesTOA > 65° F.Outdoor air temperature exceeds 65° F.Differential1b, 2b, 3b, 3c, 4b, 4c,TOA > TRAOutdoor air temperature exceeds return airDry Bulb5a, 5b, 5c, 6a, 6b, 7, 8temperatureFixedAllhOA > 28 Btu/lbOutdoor air enthalpy exceeds 28 Btu/lb ofEnthalpydry aira)ElectronicAll(TOA, RHOA) > AOutdoor air temperature/RH exceeds theEnthalpy“A” set-point curveb)DifferentialAllhOA > hRAOutdoor air enthalpy exceeds return airEnthalpyenthalpyDew-pointAllDPOA > 55° F. orOutdoor air dry bulb exceeds 75° F. orand dry-bulbTOA > 75° F.outside dew point exceeds 55° F.(65 gr/lb)temperaturesa)At altitudes substantially different than sea level, the Fixed Enthalpy limit shall be set tothe enthalpy value at 75° F. and 50% relative humidity. As an example, at approximately 6000 ft elevation the fixed enthalpy limit is approximately 30.7 Btu/lb.b)Setpoint “A” corresponds to a curve on the psychrometric chart that goes through a point at approximately 75° F. and 40% relative humidity and is nearly parallel to dry-bulb lines at low humidity levels and nearly parallel to enthalpy lines at high humidity levels.
To use Table 1, the installer has to know what sensor control type is being used in the system. This is listed in the 1st column. Listed in the 2nd column are the approved climate zones where these sensor control types can be used. The location of the climate zones are shown on a map, as illustrated in FIG. 1 for the USA, for example. The 3rd column indicates the allowed equations for the sensor types used. These equations set the high limit shut-off. This shut-off requires selection or parameterizing in the field.
The map in FIG. 1 (climate map and climate zones based in part on Köppen climate classification) shows not only the hot, moderate, and cold climate zones, but also indicates whether the zone is dry (B), moist (A), or very moist, i.e. marine (C). The setting of this system does not only determine when (seasonally) one should not bring hot air into a space to be cooled, but also when not to bring air that may be very moist. The moist air (humidity) will not only cause discomfort, but can result in conditions within the building that are suitable for mold growth, or provide other unhealthy air quality situations.
In a required climate zone setting according to the ASHRAE 90.1-2007 Energy Standard, all air economizers shall be capable of automatically reducing outdoor air intake to the design minimum outdoor air quantity when outdoor air intake will no longer reduce cooling energy usage. High-limit shut-off control settings shall be those listed in Table 1.
State of the art control devices require detailed review of the example table above by the installing or operating personnel to determine the correct set point, in order to meet the local law for the climate zone where the air conditioning economizer system is installed. Then the individual must select a corresponding operating range, e.g. through use of selector switch(es). If the individual does not have in his possession the tables and listing of climate zones, then the system can be incorrectly set up and result in energy waste and bad building conditions in moist climates. Multiple studies have proven this possibility to be true.